I think Japan might be secretly working with the NK government to build their own nuclear arsenal, but will be quick to backstab Korea and America like they had always been doing once they get a technological edge they have completely copied from the Americans by fawning to them their secrets

The trick is to allow Russian Far East and Mongolia to develop in the same route so Japan has more adversaries to contend with and balance. Japan has to be forced to become a valuable ally, or they can seek the path of their own slow internal destruction

1. Japan and North Korea really, really hate each other - far more so than Japan and the South.
2. Japan has uranium and lots of know-how. They could easily build nukes on their own.
3. Even if they needed (or wanted) foreign help, why would they want barely functional, hopelessly antiquated North Korean tech?
4. Everyone knows that Japan's superweaponry of choice would be giant robots